Method for increasing photosensitivity of photoconductive materials



Dec 18 1962 B. L. SHELY METHOD FoR INCREASING PRoTosENsIIIvITY 3,069,365

0F PHoTocoNDUcTIvE MATERIALS Filed June 3, 1960 r r 0 Z3 5 4 n w z o H L w F. D f 3 M U 5 r M00/ a M 6. i 4. /r//Z F m Mm m 0 wu -Q CK SR S@ /m/mm/Q 5), 55m/AM l.. .Sway

United States Patent Otlice 3,009,365 Patented Dec. 18, 1952 3,069,365 METHOD FOR INCREASING PHGTOSENSHTWTY PHTOCNDUCTIVE MATERIALS Benyarnrn L. Shelly, Mahtomedi, Minn., assignor to Minnesota Mining and Manufacturing Company, St. Pani,

Minn., a corporation of Delaware Filed June 3, 1960, Ser. No. 33,765 14 Claims. (Cl. 252-501) This invention relates to a method for increasing the light sensitivity and response rate of photosensitive materials. In one aspect it relates to an improved process for the dye sensftization of photoconductive materials. In another aspect it relates to a process for increasing the spectral response characteristics of a photoconductive material.

The use of photosensitive sheet materials having surface layers which become electrically conductive when irradiated with light of certain wave lengths is known. Thus, a photoconductive material such as zinc oxide can be coated on a sheet of an electrically conductive ma-` terial, exposed to a light image, andrsubjected to electrolysis in the presence of various developer solutions, such as an aqueous silver nitrate solution. The differed' by addition of the dyes to a dispersion of zinc oxide in a.

resin solution before depositing on an electrically conductive backing or by ball milling with the zinc oxideresn mix. However, the spectral sensitivity still requires the use of relatively high light intensities and/or long exposures to reproduce a visible image.

It is therefore an object of this invention to improve the sensitivity and response rateof a photoconductive material.

Another object of this invention is to provide a process for increasing the optical sensitivity of a photoconductive material.

Still another object of this invention is to improve the sensitivity of a photoconductive copy sheet.

' Yet another object of this invention is to provide a simple. eicient and economical method for improving the spectral response of a photoconductive material.

. In accordance .with this invention, the optical or light sensitivity and photoresponse characteristics of a photoconductive material, such as zinc oxide and indium oxide, are improved by dark adapting the material and subsequently treating the photoconductive particulate material in the dark with a mixture of an optical sensitizer and a partial solvent for said optical sensitizer. By partial solvent is meant an organic liquid in which the optical sensitizer employed is only slightly soluble, most preferably of the order of magnitude of *2 to l0*6 moles of dye per liter at 20 C., said organic liquid having a dielectric constant as hereinafter defined.

As mentioned earlier, the use of optical sensitizers, i.e. organic compounds which give up electrons when excited by light of certain wavelengths, is known. The diiculty in their utilization has been to obtain adequate concentrations of the sensitizer at the surface of the photoconductive material. It has now been found that a period of dark adaptation, i.e.A 24 hours or more, of the photoconductive material coupled with the step of incorporating the optical sensitizer into the photoconductive material in a dark environment greatly increases the optical sensitivity and photorespo-nse of the photoconductor, often by a factor of 2 or more.

The oplical sensitivity and photoresponse characteristics of the photoconductive material are also enhanced by carrying out the step of incorporating the optical sensitizer into the photoconductive material in the presence of a partial solvent for the optical sensitizer. Although the mechanism is not fully known, it is believed that the partial solvent may function as a carrier, permitting the solubilization of reatively small quanti ies of the sensitizer, in which solubilized form absorption on the active surface sites of the photoconductive patticles is greatly facilitated. However, when the carrier or sensitizing medium is too good a solvent for the optical sensiizer it apparently tends to compete with the photocondutcive mateial for the sensitizer and thus to limit the amount of sensitizer absorbed or taken up by the photoconductive surface, or perhaps to compete with the sensitizer for dyeable sites on the photoconductive particles. Irrespec- "i tive of the mechanism involved, the techniques of this invention produce a surprising increase in the sensitivity and rate of response of the dyed photoconductors.

Various optical sensitizers for photoconductive powders are known. Generally, oil insoluble organic dyes are most effective, particularly those dyes selected from the following classes:

Acridine dyes, e.g.: Phosphine R (46045) Xanthene dyes, e.g.:

Eosin Y (45380) Erythiosin (45430) Rose Bengal (45435) Diphenylmethane dyes, eg.: Auramine (41000) Triphenylmethane dyes, eg.:

Calcozine Violet C (42555) Erio Chrome Cyanine R (42571) Stilbene dyes, eg.:

Stilbene Yellow G (40000) Diphenyl Fast Orange 4RL (40265) Thiazoles, eg.:

Seto-avine T (49005) Primuline NAC (49000) Quinoline dyes, eg.:

Calcocid Yellow CW (47005 Calcomine Chinoline Yellow P (47035) Sulfur dyes, e.g.:

Sulfogene Bordeaux SCF (53720) Calcogene Yellow ZGCF (53160) Azo dyes, eg.:

' Azo Scarlet Y (17755) Diazophenyl Fast Scarlet G1 (17805) The dye identification numbers above are taken from the second volume of Color Index (1956). Other sensitiz`ng dyes are described in Canadian Patents Nos. 576,733 and 576,734.

The partial solvent is selected with particular regard for the speclc optical sensitizer to be used. However, generally those organic solvents having a dieelectric constant between 5 and 20, preferably between 5 and 18, are useful as partial solvents for the oil insoluble dye sensitizers. The preferred partial solvents include butyl alcohol, methyl acetate, ethyl acetate, butyl acetate, propyl acetate, methyl ethyl ketone, etc., with esters, especially acetates, having from 3 to 6 carbon atoms, being particularly preferred. Solvents such as toluene, benzene, heptane, ethyl ether, carbon tetrachloride, methyl alcohol, have been found to be unsatisfactory as partial solvents. Reagent grade materials are desirable, though not essential, since the presence of impurities can adversely edect the characteristics of the photoconductive powder.

After the photoconductive powder has been dark adapted it is suspended in the partial solvent, and the optical sensitizer-usually dissolved in a small quantity of a good solvent therefor whi:h has a dielectric constant above about 25 (e.g. methyl alcohol), which is miscibe with the partial solvent and which has better solvent properties for the optical sensitizer than the particular partial solvent with which it is to be employed-is added to the suspension. Following a thorough mixing under continued'dark conditions the powder is filtered and dried.

TheV following example is illustrative of the above techniques. 0.5 gram of Eosin Y, a xanthene dye, was completely `dissolved in 99.5 grams of ethyl alcohol. Two grams of zinc oxide powder were added to 50 ml. of ethyl acetate containing 5 drops (.25 ml.) of the above Eosin Y solution and the admixture was shaken thoroughly. The zinc oxide was filtered out and dried, and a pressed powder sample was prepared for measurement of the photoresponse characteristics. The pressed powder test was conducted as follows.

T-wo aluminum electrodes, 3 cm. wide and 0.063 cm. thick, are mounted on a Lucite block. The electrodes are parallel spaced in such a fashion as to create a sample holder therebetween, which sample holdercavity has a flatbed andxperpendicular,aluminum walls and is 3 cm.

long,- and 0.63 cm. wide'.`l` Two hundred milligrams'of they photoconductiveV powderi to be tested; areplaced uniformlyiinfthecavity and compressed with atplunger of substantiallytthesamei cross-sectional area as .the cavity, using: a` pressurezof. about 204 pounds per squarezinch.`

This forms a compacted layer of photoconductive powder:

about 0.05 cm. thick whichis pressed rmly against-the aluminum electrode walls. During testing the sample holder is enclosed in a dry box containing a dessicant. A D.C. test voltage is applied across the sample. and across a series resistance, with provision made for current measurement. The series resistance is adjusted and kept at a small Value as compared to the sample resistance-,- thus impressing all but a negligible portion of the test voltage across the sample. The small voltage drop across the series resistance is amplified and recorded, and the' conductance or conductivity'then calculated. The conductivities thus calculated on the basis of the current measurement and the sample geometry are the effective conductivities which would exist if the conductivity ywere uniform throughout. The dark conductivities are calculated from the current measurements on `the unilluminated sample, and the photoconductivity values are calculated from thecurrent measurements after 5 seconds of illumination from a 200 watt tungsten light source operating at a color temperature of 3100 K. and placed at such a -distance that about .0045 vwatt per square centimeter of radiant energy falls on the sample. watt per square centimeter of this energy is inthe visible region between 0.38 and 0170 micron wavelength.

Using the above techniques, three samples of zinc oxide powderV were dye sensitized. Sample l was dark adapted andwas dye sensitized under dark conditions. Sample V2' wasldye sensitized under dark conditions without a prior period of dark adaptation. under light'conditions without a prior period of dark adaptation. FIGURE 1 shows `the effect of these techniques on a plot of light conductance versus exposure time, using the earlier mentioned light source; It was noted that dye sensitization under Vdark conditions, particularly of the dark adapted photoconductive material, produced a material with higher light conductance and with a faster rate of response. Similar results are obtained with photoconductive indium oxide powders.

The photoconductive materials produced in accordance withv this invention not only have a higher light conductwce value and a more rapid response rate but also'possess superior spectral sensitivity, i.e. are sensitive to light from a greater portion of the spectrum. This is illustrated by FIGURE 2,' wherein sample' l4 represents anuntreated About 0.001

Sample 3 was dye sensitized sample of zinc oxide, sample 2 represents a zinc oxide sample dyed with Eosin Y (as set forth earlier) under light conditions, and sample represents a zinc oxide sample similarly dyed with Eosin Y under dark conditions. The data was obtained by placing a standard pressed powder sample 3 inches from a Bausch and Lomb grating monochromator (Model 13D-2l) in which the lamp was set at high intensity with the slit completely open. The conductance of the powder was measured over a range from 3000-7000 A., and the sample was allowed to reach a saturation value at each measured wavelength, i.e. every A. Although the intensity was not theV same at the various wavelengths, thev relative effect of dye sensitization under light and dark conditions` was observed.

With the 5 second exposure technique described earlier various solvents were evaluated as partial solvents, the results of which appear in Table I.

Itis apparent fromgth'eV data obtained that'a period' off dark adaptation of the-photoconductive material prior to dye sensitization produces a significant improvement in It is' the light'sensitivity or lightconductance values. also app arent'that"dye sensitization under dark'condi-.. tionsenhances the'- light sensitivity. sensitivity and photoresponse characteristics, particularly the rate of response, are significantly increased lwhen the optical sensitizeror dye yis'incorporatedI into the photoconductive powder in the.presence.of a partial solvent havingja dielectric constant between'S andv 20, as herein defined.

Variations and modificationswill become apparent tof" persons skilled in the art from this disclosure,landthe illustrative examples are intended onlyto exemplify the best vembodiment of thisinvention and not to..limit'the scope thereof.'

l claim:

1. A method for increasing the light`sensitivity and photoresponse characteristics of a photoconductive powder selected from the groupV consisting of zinc oxide and' indium oxide which comprises dark4 adapting said photoconductive powder for at least 24 hours, contacting said photoconductive powder in the absence of light with an optical sensitizer therefor selected from the oil insoluble organic dye group consisting of acridine dyes, xanthene dyes, diphenylmethane dyes, triphenyl methane dyes, stilbene dyes, thiazole dyes, quinolinev dyes, sulfur dyes and azo dyes, in a partial solvent media, said partial solvent being a liquid organic compound capable of dissolving only between about 10-2 and- 10-6 moles of said optical sensitizer per liter at 20 C. and having a dielectric constant between 5 and 18, and subsequently drying saidv photoconductve powder.

2. The meth-od of claim 1 wherein said photoconductive material is zinc oxide powder.

3. The method of claim` 1 wherein said photoconductive material is indium oxide powder.

Furthermore, the

4. A method for increasing the light sensitivity and photoresponse characteristics of zinc oxide powder which comprises contacting said zinc oxide under dark conditions with an organic oil insoluble optical dye sensitizer therefor in a partial solvent media, said partial solvent being a liquid organic compound with a dielectric constant between about 5 and about 20 and being capable of dissolving only between about 2 and 10-6 moles of said optical sensitizer per liter at 20 C.

5. A method for increasing the light sensitivity and photoresponse characteristics of photoconductive indium oxide powder which comprises contacting said photoconductive lindium oxide under dark conditions with an organic oil insoluble optical dye sensitizer therefor in a partial solvent media, said partial solvent being a liquid organic compound with a dielectric constant between about 5 and about 20 and being capable of dissolving only between about 10-2 and 10-6 moles of said optical sensitizer per liter at 20 C.

6. A method for increasing the light sensitivity and photoresponse characteristics of a photoconductive material selected from the group consisting of zinc oxide and indium oxide which comprises contacting said photoconductive material in the absence of light with an organic oil insoluble optical dye sensitizer therefor in a partial solvent rnedia, said partial solvent being a liquid organic compound with a dielectric constant between about 5 and about 20.

7. A method for increasing the light sensitivity and photoresponse characteristics of a photoconductive material selected from the group consisting of zinc oxide and indium oxide which comprises dark adapting said photoconductive material for at least 24 hours and thereafter contacting said photoconductive material in the absence of light with an organic oil insoluble optical dye sensitizer therefor in a partial so-lvent media, said partial solvent being a liquid organic compound with a dielectric constant between about 5 and about 20 and being capable of dissolving only between about 10-2 and l0-s moles of said optical sensitizer per liter at 20 C.

8. A method for increasing the light sensitivity and photoresponse characteristics of a photoconductive powder selected from the group consisting of Zinc oxide and indium oxide which comprises dark adapting said photoconductive powder for at least 24 hours, contacting said photoconductive powder in the absence of light with an organic, oil insoluble optical dye sensitizer therefor in a partial solvent media, said partial solvent being a liquid organic compound capable of dissolving only between about l0*2 and 10*6 moles of said optical sensitizer per liter at 20 C. and having a dielectric constant between about 5 and about 20, and subsequently drying said photoconductive powder.

9. The process of claim 8 in which the partial solvent is an ester having from 3 to 6 carbon atoms,

l0. The process of claim 8 in which the partial solvent is ethyl acetate.

11. The process of claim 8 in which the partial solvent is methyl acetate.

12. The process of claim 8 in which the par-tial solvent is butyl acetate.

13. The process of claim 8 in which the partial solvent is butyl alcohol.

14. The process of claim 8 in which the partial solvent is methyl ethyl ketone.

References Cited in the le of this patent UNITED STATES PATENTS 2,937,944 Van Dorn et al May 24, 1960 FOREIGN PATENTS 576,734 Canada May 26, 1959 587,297 Canada Nov. 17, 1959 811,165 Great Britain Apr. 2, 1959 OTHER REFERENCES Putseiko et al.: sensitization of the Internal Photoeffect of Semiconductors by Chlorophyll and Allied Pigments, translated from Doklady Akademii Nauk., SSSR, 90, 1005-08 (1953) NSF-Tr-147, December 1953. 

1. A METHOD FOR INCREASING THE LIGHT SENSITIVITY AND PHOTORESPONSE CHARACTERISTICS OF A PHOTOCONDUCTIVE POWDER SELECTED FROM THE GROUP CONSISTING OF ZINC OXIDE AND INDIUM OXIDE WHICH COMPRISES DARK ADAPTING SAID PHOTOCONDUCTIVE POWDER FOR AT LEAST 24 HOURS, CONTACTING SAID PHOTOCONDUCTIVE POWDER IN ADSENCE OF LIGHT WITH AN OPTICAL SENSITIZER THEREFOR SELECTED FROM THE OIL INSOLUBLE ORGANIC DYE GROUP CONSISTING OF ACRIDINE DYES, XANTHENE DYES, DIPHENYLMETHANE DYES, TRIPHENYL METHANE DYES, STILBENE DYES, THIAZOLE DYES, QUINOLINE DYES, SULFUR DYES AND AZO DYES, IN A PARTIAL SOLVENT MEDIA, SAID PARTIAL SOLVENT BEING A LIQUID ORGANIC COMPOUND CAPABLE OF DISSOLVING ONLY BETWEEN ABOUT 10-2 AND 10-6 MOLES OF SAID OPTICAL SENSITIZER PER LITER AT 20*C. AND HAVING A DIELECTRIC CONSTANT BETWEEN 5 TO 18, AND SUBSEQUENTLY DRYING SAID PHOTOCONDUCTIVE POWDER. 